Guide to File Systems in Operating Systems

Guide to Operating Systems,
4th
ed.
Chapter 4: File Systems

Guide to Operating Systems, 4th ed. 2
Objectives
2
• List the basic functions common to all file systems
• Explain the file systems used by Windows XP,
Windows Server 2003, Windows Vista, Windows
Server 2008, and Windows 7 (FAT16, FAT32,
FAT64, and NTFS)
• Discuss the file systems used by UNIX and Linux
systems, including ufs and ext
• Explain the Mac OS X Extended (HFS+) file system
including new features added in Mac OS X version
10.6 Leopard

Understanding the File System Functions
• All information stored on a computer’s hard disk is
managed, stored, and retrieved through a file
system
– The file system allocates locations on a disk for storage and it
keeps a record of where specific information is kept
– Some file systems also implement recovery procedures when a
disk area is damaged or when the OS goes down
• The overall purpose of a file system is to create a
structure for filing data
• A file is a set of data that is grouped in some logical
manner, assigned a name, and stored on the disk

Understanding File System Functions
• File systems used by operating systems perform
the following general tasks:
– Partition and format disks to store and retrieve information
– Enable files to be organized through directories and folders
– Establish file-naming conventions
– Provide utilities to maintain and manage the file system and
storage media
– Provide for file and data integrity
– Enable error recovery or prevention
– Secure the information in files

Understanding File System Functions
• Directory or folder – organizational structure that
contains files and may additionally contain
subdirectories (or subfolders)
• Directories may store the following information
– Date and time the directory or file was created
– Date and time the directory or file was last modified
– Date and time when the directory or file was last accessed
– Directory or file size
– Directory or file attributes, such as security information, or if the
directory or file was backed up
– If the information in a directory or file is compressed or
encrypted

Designing a Directory Structure
• A chaotic file structure:
– makes it difficult to run or remove programs
– Makes it difficult to determine the most current versions
– Makes users spend unproductive time looking for specific files
• Some users keep most of their files in the computer’s
primary level or root directory (root folder)
• Some programs use an automated setup that suggests
folders for new programs
– Example – creating new subfolders under the Program Files folder
• To avoid chaos, design the file and folder structure from
the start (especially on servers)

• Consider following some general practices:
– Root folder should not be cluttered with files or too many
directories/folders
– Each software application should have its own folder/subfolder
– Similar information should be grouped (example: accounting
systems or office productivity software)
– Operating system files should be kept separate and protected
– Directories and folders should have names that clearly reflect
their purposes (a folder named “Shared” would contain
documents that can be shared by many users)

Sample folder structure for a Windows-based system

Disk Storage Basics
• Hard disks arrive from manufacturer with low-level
formatting
– A low-level format is a software process that marks the
location of disk tracks and sectors
– Tracks are like several circles around a disk and each track is
divided into sections of equal size called sectors

Disk Storage Basics
Disk tracks and sectors on a platter

Block Allocation
• Block allocation – divides the disk into logical blocks
called clusters, which correlate to sectors, heads, and
tracks on the disk
– Keeps track of where specific files are stored on the disk
• Cylinders – tracks that line up on each platter from
top to bottom and are all read at the same time
• Data regarding block allocation is stored using one
of two techniques:
– File allocation table (FAT)
– New Technology File System (NTFS) and Unix/Linux file systems
– uses various locations on the disk to store a special type of file
that is used for directory and file allocation information

Partitions
• Partitioning – process of blocking a group of
tracks and sectors to be used by a particular file
system, such as FAT or NTFS
• After partitioning, the disk must be high-level
formatted in order for the OS to store files
• It might be necessary for a disk to have more than
one file system
– Will require a partition for each file system
– Example: To allow the installation of Red Hat Enterprise Linux
and Windows 7 on the same computer

Partitions
• Logical drives – creating multiple logical volumes
on a single disk and assigning drive letters to each
volume
• When a partition is created, information about that
partition is stored in a special area of the disk
known as the partition table (in MS-DOS, Mac
OS, and Windows) and disk label (in UNIX/Linux)
• Another piece of disk that is reserved is known as
the boot block in UNIX/Linux and Mac OS X, or
the Master Boot Record (MBR) in MS-DOS and
Windows.

Partitions
• In Windows, the MBR consists of four elements:
– The boot program – examines the partition table to determine
which partition to boot from
– The disk signature – stores information about the disk and is
used by management software such as the Windows registry
– The partition table for the active partition
– The end-of-MBR marker – where the MBR ends
• Not all operating systems support partitions in the
same way
• Each operating system uses specific utilities to
create partitions

Formatting
• Formatting – the process of placing the file system
on the partition
– Necessary in order to install an operating system
• After the OS is installed, a disk management tool
can be used to partition and format additional free
space
• Can also use the format command from the
Command Prompt window
– The format command includes several switches (extra codes)
– To view a list of these switches, type format /?

Formatting
• When a file is stored to disk:
– Data is written to clusters on the disk
– Filename is stored in the folder, along with the number of the first
cluster the data is stored in
– When the OS fills the first cluster, data is written to the next free
cluster and the FAT entry corresponding with the first cluster points to
the number of the second cluster used
– When the second cluster is full, the OS continues with the next free
cluster and the FAT entry for the second cluster points to the number
of the third cluster used, and so on…
– When a file is completely written to the disk, the FAT entry for the final
cluster is filled with all 1s (means end of file)
– This is commonly referred to as the linked-list method

Formatting
• Clusters are a fixed length
– When a files does not use all of the space in a cluster, the rest
of the cluster is unusable
– Unusable spots are marked in the FAT as bad clusters (never
used for file storage)
• Each partition stores and extra copy of the FAT
table in case the first copy gets damaged
• There is only one copy of the root directory on each
partition (see figure on the following slide)
• The FAT tables and root directory are at the
beginning of each partition and always at the same
location

Formatting
Typical FAT directory structure

Formatting
• Each FAT directory entry contains filename, file
change date and time, file size, and file attributes
• Attributes can indicate whether a file is set to:
– Read-only
– Archive (to be backed up the next time a backup is made)
– System
– Hidden

Windows File Systems
• Windows XP, Vista 7, Server 2003, and Server
2008 support three files systems:
– Extended FAT16
– FAT32
– NTFS
• These OSs also support file systems for DVD/CD-
ROM drives and USB devices (flash drives)

FAT16 and Extended FAT16
• Extended FAT16 evolved from FAT16 used in
earlier versions of MS-DOS and Windows
(3.x/95/98/Me)
• In extended FAT16:
– Maximum size of a volume is 4GB
– Maximum size of a file is 2GB
– Has been around for awhile and can be read by non-Windows
operating systems like UNIX/Linux
– Considered a stable file system
– Long filenames (LFNs) can be used
• Can contain up to 255 characters
• Not case sensitive

FAT32
• Support for FAT32 started with Windows 95
Release 2
• Designed to accommodate larger capacity disks
• FAT32:
– Root folder does not have to be at the beginning of a volume
– Can use disk space more efficiently than FAT16 (because it
uses smaller cluster sizes)
– Largest volume that can be formatted is 32 GB
– Maximum file size is 4 GB
– Offers fast response on small 1 or 2 GB partitions

FAT64
• FAT64 is also known as exFAT
• Proprietary file system introduced by Microsoft for
mobile personal storage
• Good choice for USB flash devices that may store
large files (such as pictures, videos, etc…)
• Available in Service Pack 1 for Windows Vista,
Windows 7, and Windows Server 2008, Mac OS X
Snow Leopard
• Support is available for Linux from a third party

NTFS
• NTFS – dominant Windows file system for all
Windows operating systems starting with Windows
2000
• Uses a Master File Table (MFT) instead of FAT
tables
• The MFT and related files take up about 1 MB of
disk space
• When a file is created, a record for that file is
added to the MFT
– Contains additional attributes such as security settings,
ownership, and permissions

NTFS
• The MFT record reflects the sequence of clusters
that a file uses
• It is possible to have multiple filenames that refer to
the same file
– A technique known as hard linking
– This feature is also available in UNIX/Linux file systems
• Windows Vista, Server 2008, and 7 use NTFS
version 6
• Windows XP and Server 2003 use NTFS version 5
• Windows NT 4.0 used NTFS 4

NTFS
• Basic features of NTFS:
– Long filenames
– Built-in security features
– Better file compression than FAT
– Ability to use larger disks and files than FAT
– File activity tracking for better recovery and stability than FAT
– Portable Operating System Interface for Unix (POSIX)
support
– Volume striping and volume extensions
– Less disk fragmentation than FAT

NTFS
• NTFS is equipped with security features that meet
the US government’s C2 security specifications
– Refers to high-level, “top-secret” standards for data protection,
system auditing, and system access
• Examples:
– System files can be protected so only the server administrator
has access
– A folder of databases can be protected with read access, but
no access to change data
– Public folder can give users in a designated group access to
read and update files, but not to delete files

NTFS
• Some files can be compressed by more than 40%,
saving disk storage for other storage needs
• NTFS has the ability to keep a log or journal of file
system activity (called journaling)
– Makes it possible for files to be restored in the event of a power
failure
• NTFS supports volume striping
– Process that equally divides the contents of each file across two or
more volumes to extend disk life, enable fault tolerance, and
balance disk load for better performance
• NTFS has hot fix capabilities
– If a bad disk area is detected, automatically copies the information
to another disk area that is not damaged

NTFS
• In addition to NTFS 4 features, NTFS 5 adds
several new features:
– Ability to encrypt files
– No system reboot required after creating an extended volume
– Ability to reduce drive designations
– Indexing for fast access
– Ability to retain shortcuts and other file information when files
and folders are placed on other volumes
– Ability to establish disk quotas (to control how much disk space
users can occupy)

NTFS
• NTFS 6 (latest version) adds several new features:
– Transactional NTFS – used to perform operations in
transactions (all at once or not all)
– Partition resizing – allows administrator to expand or shrink
partitions
– Self-healing – the chkdsk utility runs in the background to
correct hard disk problems (instead of having to take the
volume down to run it)

NTFS
• The chkdsk utility can detect and fix an extensive
set of file system problems in FAT and NTFS
system
– This utility is available in all versions of Windows starting with
Windows 2000
– The most common problems found are files with 0 sizes,
caused when a file is not properly closed or chains of clusters
that have no directory entries
• Windows 2000 and later versions have a built-in
disk defragmenting tool
– Rearranges data on the disk in a continuous fashion, ridding
the disk of scattered open clusters

NTFS
FAT16, FAT32, FAT64, and NTFS compared

CDFS and UDF
• Windows versions after Windows 2000 recognize
some additional file systems used by peripheral
storage technologies
• CD-ROM File System (CDFS) – supported so that
OSs can read and write files to DVD/CD-ROM
drives
• Universal Disk Format (UDF) – also used on
DVD/CD-ROMs, which are used for huge file
storage to accommodate movies and games

The UNIX File System
• There are many different file systems that can be
used with UNIX
– Some file systems are more “native” to specific UNIX operating
systems than others
• Most versions of UNIX and Linux support the UNIX
file system (ufs), which is the original native UNIX
file system
– ufs is a hierarchical file system that is expandable, supports
large storage, provides excellent security, and is reliable
– Many qualities of NTFS are modeled after ufs (journaling and
hot fixes)

• In Linux, the native file system is called the
extended file system (ext or ext fs)
• ext is modeled after ufs and enables the use of the
full range of built-in Linux commands, file
manipulation, and security
• The first ext version had bugs
– ext2 – reliable file system that handles large disk storage
– ext3 – added journaling capabilities
– ext4 – supports file sizes up to 16 TB

• Both ufs and ext use the same structure
– Built on the concept of information nodes (or inodes)
– Each file has an inode and is identified by an inode number
– An inode contains general information about that file such as:
• User and group ownership, permissions, size and type of file, date
the file was created, and the date the file was last modified and
read
– Each disk is divided into logical blocks
• The superblock contains information about the layout of blocks,
sectors, and cylinder groups on the file system
– The inode for a file contains a pointer (number) that tells the
OS where to find a file on the hard disk (based on logical
blocks)
– Inode 0 contains the root of the folder structure and is the
jumping off point for all other inodesGuide to Operating Systems, 4th ed. 36

• A UNIX/Linux system can have many file systems
– Mounts file systems as a sub file system of the root file system
– All file systems are referred to by a path
– The path starts out with / (/ indicates the main root directory of
the file system)
– See Figure 4-14 on the next slide for examples
• Directories in the file system contain a series of
filenames
– UNIX/Linux allows the use of long filenames, which may
include any character that can be represented by ASCII

UNIX/Linux file system path entries

• Disks are referenced by a special inode called a
device
• There are two types of devices:
– Raw device – has no logical division in blocks
– Block device – does have logical division in blocks
• Devices are normally kept in the /dev or /devices
directory
• Symbolic link – used to link a directory entry to a
file that is on a different partition
– Merely a pointer to a file

• You must first partition a disk to use the UNIX/Linux file
system
• The command to partition the disk differs slightly
– Most UNIX systems use either fdisk or format
– Typing man fdisk or man format at the command prompt gives you an
overview of available commands

• Once a partition is made, a file system can be
created
– You must know the device name of the partition on which you
wish to create a file system
– Type newfs, followed by the name of the device
– The newfs command is not available in all versions of Linux
• Use the mkfs command instead

• When a file is saved to disk, the system first stores
part of the data to memory until it has time to write
to disk
– If computer is shut down prior to data being written to disk, you
can end up with a damaged file system
– UNIX/Linux systems should always be shut down using proper
shutdown commands
– You can manually force a write of all data in memory by using
the sync command
• Another utility is known as fsck
– Verifies the integrity of the superblock, the inodes, all cluster
groups, and all directory entries

UNIX/Linux file system commands

The Macintosh File System
• The original Macintosh Filing System (MFS) of
1984 was limited
– Kept track of 128 documents, applications or folders
– Reasonable limit when the only storage device was a 400 KB
floppy disk drive
• In 1986, Apple created Hierarchical Filing System
(HFS)
– Divided a volume (disk partition) into allocation blocks – similar
to clusters on PCs
• In 1998, Apple released Mac OS 8.1 with a new file
system called Hierarchical Filing System Extended
Format (HFS+)

• In Mac OS X version 3.0, Mac OS Extended
(HFS+) includes new features:
– A case-sensitive format to make the file system more
compatible with other UNIX/Linux systems
– Journaling
– Ability to store up to 16 TB of data
• The first two sectors of a Mac-formatted disk are:
– Boot sectors, or boot blocks
– Volume information block – contains catalog b-tree
• Catalog b-tree is a list of all files on the volume and keeps track of
a file’s name, location in the folder structure, and its physical
location

• Macintoshes can read and write to disks from other
operating systems
• Mac OS has always supported what might be
called medium filenames (up to 31 characters)
– Any character may be used in a filename except the colon (:)
– Reason for this: Macintosh paths are written as colon-
separated entities such as:
• Hard Drive:System Folder:Preferences:Finder Prefs
• Mac uses type codes and creator codes instead of
filename extensions as in Windows
– Files created with Apple’s SimpleText editor have a type code
of APPL and a creator code of ttxt

• Macintosh files can contain two parts, or forks:
– Data fork – contains frequently changing information (such as
word processing data)
– Resource fork – contains information that is fixed (such as a
program’s icons, menu resources, and splash screens)
• Because Mac files have type and creator codes
and two forks can create problems when storing
files on non-Macintosh servers
– MacBinary – format which joins the two forks into one, and
safely stores the type and creator codes and finder flags

• Apple’s equivalent to a Windows shortcut is the
alias (introduced in System 7.0 in 1991)
– The system-level Alias Manager keeps track of the original
– The word “alias” is tacked onto the filename when the alias is
created and the filename is italicized
• Mac OS X comes with two disk utilities:
– Disk Utility – manages disk drives
– Disk First Aid – repairs disk problems
• Mac OS X also comes with a utility (Sherlock) that
searches disks for filenames and text within files
– Spotlight replaced Sherlock in Mac OS X 10.4 Tiger

A Spotlight search in Mac OS X Snow Leopard

• When a Mac is not shut down properly a disk
integrity check will automatically run at the next
startup
• Macs will boot from a DVD/CD-ROM or various
SCSI devices
– To boot from a DVD/CD-ROM, press the “C” key while booting
up
– Pressing the Shift-Option-Delete-Apple (SODA) keys while
booting will bypass the internal hard drive and boot from the
next drive in a SCSI chain

Chapter Summary
• For the user, files are the “bread and butter” of an operating
system. Besides containing the operating system kernel, files hold
documents and programs on which users rely.
• Files are made possible by a file system that enables them to be
created, written, managed, and stored on disk media. All OSs must
have a file system that provides a file-naming convention, a way to
store files, and a means to partition and format disks.
• Besides creating and modifying files, the file system also should
offer the ability to defragment files, compress file contents, ensure
file and data integrity, secure files, and control removable storage
media.
• The main file systems used in Windows since Windows 2000 are
extended FAT16, FAT32, and NTFS.

Chapter Summary
• In FAT16 and FAT32, the file system creates a file allocation
table to store information about files.
• FAT32 is more robust than FAT16, providing for the use of
more clusters and larger partitions.
• FAT64 (exFAT) is mainly used for personal mobile storage
devices like flash drives.
• NTFS is the native file system for Windows 2000 and after
with the advantage of better security, larger disk and file
sizes, better management tools, and greater stability than
FAT16 and FAT32.
• chkdsk is an important disk verification and repair utility that
works for FAT16, FAT32, and NTFS.

Chapter Summary
• UNIX and Linux support many support many different file
systems but typically employ ufs or ext
• ufs and ext use information nodes (inodes) to organize
information about files.
• Different varieties of UNIX/Linux use different file system
utilities, such as fdisk and format to partition and format
disks. The fsck (file system checker) utility is used to verify
the integrity of UNIX/Linux file systems.
• Mac OS X uses Mac OS Extended (HFS+) file system, which
is an enhancement of HFS and was introduced in 1998 with
Mac OS 8.1.

Chapter Summary
• Two important Mac OS X disk tools include Disk Utility and
Disk First Aid.
• Table 4-8 shows the disk management tools you have
learned about in the chapter. Of these tools, the Mac OS X
tool might be the most “one-stop,” because you can use it to
configure, repair, and manage disks.

Guide to File Systems in Operating Systems

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